Relatively short chopped fibrous glass strands are frequently shipped in cartons for further processing in the manufacture of other products. Equipment designed to handle relatively short chopped strands can be deleteriously affected when called upon to handle relatively long strands inadvertently included in the cartons. Occurrence of relatively long strands in the intended production of relatively short chopped glass strands indicates that the chopper needs adjustment. Formerly, visual inspection of the chopped strands in a carton was relied upon to detect the presence of chopped strands that were too long. Sometimes the long chopped strands were not detected immediately, and occasionally cartons containing some long strands were made ready for shipment before discovery of long strands in the carton then being filled.